Electric heating elements have long been made with a tubular metal sheath and although they have been and still are, commercially acceptable, the metal sheath has caused a multitude of problems. First of all, the metal from which the sheath is made is expensive, and this is particularly true when the sheath is formed of copper or copper alloy, or of stainless steel or other corrosion resistant metals. Further, the formation of the metal tube is comparatively expensive, whether it be seamless or formed by shaping flat metal into tube form and welding the longitudinal split.
In addition to the above, the metal tubular sheath required careful cleaning operations to insure that no deleterious matter contaminated the powdered magnesium oxide which electrically insulated the heating resistor from the sheath and conducted heat from the resistor to the sheath. The magnesium oxide also presented considerable problems since it has to be of high purity and free of contaminants. This required that expensive testing procedures be established and constantly followed.
Further, in use in heating liquids, such as water in a hot water tank, galvanic currents were set up between the sheath and any exposed metal surface of the tank, and such currents caused corrosion of the various anodic metallic components in the system, such as exposed tank surfaces or the protective magnesium anode.
Although many experts have held that it could not be done, I have discovered that the sheath of the electric heating element could be formed of a thermoplastic material and successfully manufactured and operated. This has overcome the many disadvantages of the metal sheathed element, above-noted. First of all, plastic tubing at the present time is considerably less expensive than the metal sheaths heretofore used.
No careful cleaning of the inside wall surface of a plastic sheath is required and only a low grade of magnesium oxide, or any other compactable low cost material may be used, since it need only have minimal electrical insulating properties to support turn-to-turn voltages and the property to compact and to conduct heat from the resistor to the sheath. There is little or no scale build-up on the exterior of the plastic sheath as there is on a metal sheath, and the sheath, being plastic and therefore non-conductive, does not build up a galvanic cell in use, as in the case of a metal sheathed unit. Since there is no voltage stress from the resistor to the plastic sheath, because the latter is non-conductive, all the heretofore required high potential tests for current leakage from the resistor to the sheath are eliminated.
The mounting block for supporting the electric heating element of my invention is also preferably formed of a thermoplastic material, as is the terminal block and the end bushing within the sheath. All of these parts may be easily sealed to each other by use of adhesives, heat sealing or sonic welding, to further provide operating advantages and economies in manufacture.